The prior art discloses numerous sound pick-ups, which are embedded in an instrument bridge. For example, U.S. Pat. No. 4,189,969 discloses a pick-up structure with individually potted, piezoelectric sensor elements having a T-shaped slot for receiving interchangeable top elements in contact with the strings. German Patent No. 3,536,921 discloses an elongated, shielded structure of piezoelectric crystals, which are arranged within a nut portion. Various other constructions are disclosed in U.S. Pat. Nos. 3,154,701, 4,252,990, 4,278,000, 4,290,331, 4,378,721, 4,380,357, and 4,160,401, as well as German Patent 3,613,888.
The basic construction problem for any sound pick-up is to produce a true-to-nature signal and a good signal-to-noise ratio. This problem acquires special dimensions in a piezoelectric pick-up, because the latter is a so-called "contact" pick-up. The signal is produced by the direct action of pressure waves, which are transmitted by one or more piezoelectric crystal elements via a coupling structure such as the bridge nut. Thus, both the geometry and the mechanical-acoustic characteristics of the coupling structure become very important. Ideally, the transmission of the vibration energy of the string via the coupling structure to the sensor element should take place with maximum sound fidelity and high efficiency.
In practice it has been found that the known piezoelectric pick-ups do not completely satisfy these requirements as evidenced by the fact that each manufactured product has its "own sound". Often, for design reasons, deep resonances are filtered out, which leads to shrill sounds with over-accentuated and noisy string side-tones. Constructions with a pick-up unit for several strings together tend to transmit sound from the individual strings at different levels.
Modern electronic sound processing such as MIDI (Musical Instrument Digital Interface) or multi-channel sound recording requires from each string a separate signal with high cross talk attenuation (.gtoreq.40 dB) with respect to the adjacent strings.
Additional problems are caused by the use of a pick-up in acoustic instruments. Even before reaching an adequate amplification level, feedback frequently occurs produced by the reactive excitation of the resonant cavity by the sound waves of the loudspeaker. Traditional and "exotic" string instruments, such as e.g. harps or the Chinese cheng require a mechanically separate pick-up subassembly for each string because the relevant string spacing varies from instrument to instrument.